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Measurement is required for developing lithographic processes and is fundamental to engineering and manufacturing control. The problem of accurate measurement is particularly significant for lithographers, where the extremely small features of state-of-the art processes have pushed measurement capabilities to their limits, and, some might say, beyond. Some of the metrology issues and challenges faced by lithographers are discussed in this chapter. 9.1 Linewidth measurement A number of methods have been used to measure the widths of resist lines, spaces, and holes in resist. Optical methods were used in the earliest days of the semiconductor industry, but were replaced by 1990 by scanning electron microscopes as the primary tools for linewidth measurement. Consequently, these early types of optical methods are not discussed further in this book. However, for linewidth measurement, optical methods have returned in a new form, scatterometry, and this technique is discussed. A number of other methods for measuring linewidths are used in critical, specialized applications, and one of these, electrical linewidth measurement, is also included for discussion. 9.1.1 Linewidth measurement using scanning electron microscopes The most common tool for measuring linewidths is the scanning electron microscope (SEM). Low-voltage SEMs are capable of measuring resist features in-line. In the scanning electron microscope, electron beams are scanned across patterns on wafers. The voltage of the electron beams ranges from a few hundred volts to tens of thousands of volts. For measuring resist features, a typical range is 300–1000 volts. The incident beam is scattered, both elastically and inelastically producing secondary and back-scattered electrons. By commonly accepted definition, secondary electrons are those with energy less than 50 eV, while the back-scattered electrons are those with energies closer to that of the incident beam. The secondary electrons created by inelastic scattering or the elastically back-scattered electrons are detected synchronously with the scan of the incident beam (Fig. 9.1). Because the number and direction of the scattered electrons depend upon the material composition and topography of the features over which the incident beam is scanned, the intensity of the detected signal varies so that an image can be created.
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